1. Field of the Invention
The present invention generally relates to a copper base alloy and a method for producing the same. More specifically, the invention relates to a copper base alloy having an excellent hot workability, which is used as the material of electric and electronic parts, such as connectors, and a method for producing the same.
2. Description of the Prior Art
In recent years, with the development of electronics, the complication and integration of electric wiring for various machines is advanced to increase the amount of wrought copper and copper-alloys used as the materials of electric and electronic parts, such as connectors. In addition, it is required to decrease the weight and production costs of electric and electronic parts, such as connectors, and it is required to enhance the reliability thereof. In order to meet these requirements, copper alloy materials for connectors are thinned and pressed in complicated shapes, so that the strength, elasticity, conductivity, bending workability and press moldability thereof must be good.
Phosphor bronzes containing tin (Sn) and phosphorus (P) in copper (Cu) have excellent characteristics, such as excellent spring characteristic, workability and press punching quality, and are utilized as the materials of many electric and electronic parts, such as connectors. However, it is required to decrease production costs of phosphor bronzes, and it is required to improve conductivity thereof. In addition, phosphor bronzes have a bad hot workability to be easily broken if hot-worked, so that a plate of a phosphor bronze is usually produced by repeating homogenization, cold rolling and annealing of an ingot having a thickness of about 10 to 30 mm, which is obtained by the horizontal continuous casting. Therefore, the improvement of the hot workability of phosphor bronzes can greatly contribute to a decrease in production costs of phosphor bronzes. As methods for improving the hot workability of phosphorbronzes, there have been proposed methods for improving the hot workability of phosphor bronzes by setting predetermined temperature and working conditions during hot working (see, e.g. Japanese Patent Laid-Open Nos. 63-35761 and 61-130478), and methods for improving the hot workability of phosphor bronzes by adding iron (Fe), nickel (Ni), cobalt (Co) and manganese (Mn) for improving the hot workability and by controlling the amount of elements for inhibiting the hot workability so that it is a very small amount (see, e.g. Japanese Patent Laid-Open No. 2002-275563).
In addition, brasses containing zinc (Zn) in copper (Cu) have excellent characteristics, such as excellent workability and press punching quality and low costs, and are utilized as the materials of many electric parts, such as connectors. However, it is required to further improve the strength, spring characteristic, stress relaxation resistance and stress corrosion cracking resistance of brasses in order to cope with the miniaturization of parts and the deterioration of working environments. In such circumstances, there have been proposed methods for improving the above described characteristics by adding a predetermined amount of tin (Sn) to a Cu—Zn alloy (see, e.g. Japanese Patent Laid-Open Nos. 2001-294957 and 2001-303159).
However, in the above described methods disclosed in Japanese Patent Laid-Open Nos. 63-35761, 61-130478 and 2002-275563, there are many constraints on production conditions and component elements. Therefore, it is required to provide a method capable of decreasing such constraints.
In addition, the above described Cu—Zn—Sn alloys disclosed in Japanese Patent Laid-Open Nos. 2001-294957 and 2001-303159 are formed as a plate having a predetermined thickness usually by a method comprising the steps of carrying out the longitudinal continuous casting, heating the obtained ingot by a heating furnace, extending the heated ingot by hot rolling, and thereafter, repeating cold rolling and annealing. Although the mechanical characteristics, such as tensile strength and 0.2% proof stress, stress relaxation resistance and stress corrosion cracking resistance of Cu—Zn—Sn alloys can be improved by the addition of Sn, it is desired to improve the hot workability thereof. That is, there are some cases where Cu—Zn—Sn alloys may be broken during hot rolling to deteriorate the surface quality and yields of products, so that it is desired to improve the hot workability of Cu—Zn—Sn alloys.
One of the reasons why the hot workability is deteriorated by adding Sn to Cu or Cu—Zn alloys is that the temperature difference between the liquidus and solidus lines of copper base alloys. Thus, Sn and Zn segregate during casting, and phases having low melting points remain during solidification. For example, phases having low melting points, such as a Cu—Sn epsilon phase, a Cu—Zn gamma phase and a phase formed by solid-dissolving Cu and/or Zn in an Sn solid solution, remain in Cu—Zn—Sn alloys. Thus, the remaining second phase is dissolved during overheating when hot rolling is carried out, so that the hot workability deteriorates. Therefore, it is required to provide a copper base alloy having a more excellent hot workability. If Sn is added to a Cu—Zn alloy, the temperature difference between solidus and liquidus lines is easy to be greater than that when Sn is added to Cu, so that it is desired to improve the hot workability.
In addition, if Mn, Al, Si, Ni, Fe, Cr, Co, Ti, Bi, Pb, Mg, P, Ca, Y, Sr, Be and/or Zr is added to a Cu—Zn alloy or Cu—Sn alloy, it can be expected to improve characteristics, such as 0.2% proof stress, tensile strength, spring limit value, stress relaxation resistance, stress corrosion cracking resistance and dezincing resistance, due to the additional element(s). However, the above described temperature difference between liquidus and solidus lines (a melting/solidification range) increases to deteriorate the hot workability, so that it is required to provide a copper base alloy capable of being more simply cast in good yield.
As an example of a method for preventing the production of cracks in a copper base alloy during hot rolling, Japanese Patent Laid-Open No. 2001-294957 has proposed a methods for preventing the production of hot cracks in a Cu—Zn—Sn alloy by restricting composition, controlling the cooling rate during melting/casting, or controlling the maximum temperature during hot rolling. However, it is desired to provide a method for more simply improving the hot workability of the copper base alloy.